(1) Field of the Invention
The present invention relates to an oxygen sensor used for measuring the oxygen concentration in the exhaust gas of internal combustion engines by using an oxygen concentration cell consisting essentially of a solid electrolyte.
(2) Description of the Prior Art
There has hitherto been widely known an oxygen sensor which is used for measuring the oxygen concentration in the exhaust gas of internal combustion engines, such as in automobiles and the like, by using an oxygen concentration cell consisting essentially of a solid electrolyte, such as zirconia or the like, and for controlling the air/fuel ratio with a high accuracy. In a conventional oxygen sensor shown in FIG. 1, a tubular solid electrolyte 1 closed at its one end and having inner and outer electrodes 2 and 3 formed on its inner and outer surfaces, respectively, is gastightly received in a housing 4. The outer electrode 3 is electrically connected to the housing 4, and the inner electrode 2 is electrically connected to a center conductor 7 through a flexible conductive material 6. The center conductor 7 is pressed towards the solid electrolyte 1 by means of a spring 8 to secure its electric contact with the inner electrode 2 and to conduct the potential of the inner electrode 2 to a connecting terminal 9 through the spring 8. Further, the connecting terminal 9 is fitted in and covered with an insulating body 11 in order to insulate electrically the connecting terminal 9 from a metallic container 10 contacted with the housing 4 having the same potential as that of the outer electrode 3. However, in the oxygen sensor having the above described structure, when an external force (often caused on handling the said oxygen sensor) is applied to the connecting terminal 9 in a direction perpendicular to the axis of the oxygen sensor, the insulating body 11 is inclined due to the presence of a space between the insulating body 11 and the metallic container 10, and a lateral force acts upon the spring 8 fitted into the concave portion of the insulating body 11, and hence contact failure occurs in the fitted portion of the center conductor 7 in the solid electrolyte to cause unstable electrical connection between the inner electrode 2 and the center conductor 7, and to cause cracks in the solid electrolyte 1 due to the stress caused by the irregular pressure applied thereto by the center conductor 7.
Furthermore, in the oxygen sensor having the above described structure, an air inlet opening 13 is formed in the metallic container 10 and the distance from the tip of a boot 12 to the said air inlet opening 13 is very short, and therefore when an automobile is splashed with water while running, water penetrates easily into the interior of the metallic container 10, and the solid electrolyte 1 is rapidly cooled and is broken. The inventors have succeeded in overcoming the above described drawbacks of conventional oxygen sensors.